Some years ago I wrote a piece for a British magazine about a Royal Navy frigate. A state-of-the-art anti-submarine warship, she displaced around 4,800 tons and carried me from the naval base at Portland down the coast to Plymouth, England, at 30 knots, thanks to two Rolls-Royce Olympus gas turbines. Mischievously, as we anchored in the sound in the gathering dusk, I asked the engineering officer to calculate his ship's fuel consumption—not in miles per gallon, but in feet per gallon. With a wry grin he duly did so, and the answer was 273. "Roughly," he added.

Two hundred and seventy-three feet per gallon. An insane statistic, obviously, and a mere footnote to the main story, which, after all, concerned a fearsome cold warrior, armed to the teeth with Sea Wolf missiles and nuclear depth charges. But just recently I hitched a ride on the first Pershing 115 as she cruised up the Italian coast at high speed, from La Spezia to the Genoa Boat Show, and I have to confess, I felt a similar desire to translate this turbocharged behemoth's prodigious appetite for diesel into everyday language. Just how far would a gallon of Kuwait's finest take her?

It seemed an unworthy thought. It's impolite to bring such subjects up with boats like this. You focus on how beautiful they are, how fast they go, and, if you know the owner well, how much they cost. But nevertheless, with two 3,700-hp V-16 diesels in the engine room and 40 knots on the log, such things do cross your mind.

Astonishingly, Pershing boss Tilli Antonelli didn't think 7,400 hp would be quite enough for some owners, so each 115 is built with a central molded-in stern pod, ready to accommodate a TF50 gas turbine. The extra 5,600 hp this brings to the party should push the 115's top speed up to around 55 knots (63 mph), with cruising speeds in the high 40s. As for what it will do for the boat's fuel consumption—well, it won't be pretty.

Even without the TF50, the Pershing 115 is quite a piece of work. Antonelli takes great pride in creating driver's boats, with exciting performance and sweet handling. And although even his sternest critic would probably cut him a little slack now that he's building boats of 100-plus tons, the gargantuan 115 turns out to be as great a ride as all her little sisters. There's that familiar dramatic heel when you crank the helm hard over; a tight turning circle; taut, assertive seakeeping; and a solid, thoroughbred feel. Acceleration is an inevitable victim of the physics of power to weight, but it's still not at all bad—have a look at that curve on the boat spped graph. As for top speed, this boat takes no prisoners.

Weird as it seems—and believe me, in a boat of this size and displacement, it did seem weird—I found the 115 almost as much fun to handle as the superb 62 I'd tested a couple of weeks before.

The 115 is a true Pershing and a great piece of engineering. Getting a boat like this right is impressive enough; getting it right the first time is extraordinary. Yet Antonelli told me that this prototype reached 42 knots the morning after launch (with a clean bottom and clean waterjets) and added, perfectly straight-faced, that they'd found the center of gravity to be four centimeters off. "But we're happy with that," he said, cracking a smile.

In fact, he was happy with most things about the 115 when she faced her public for the first time at the autumn shows, and it was easy to see why. The four-cabin layout seems conventional enough on paper, but in the flesh it's refreshingly different. The first two things you notice are how large and unencumbered the saloon is, with no bulkheads to interrupt sightlines and the galley stashed below in the crew's area. The owner of our test boat met the artist Vittoria Vanghelis as she worked on some pieces for Norberto Ferretti, boss of Pershing's parent company, and commissioned her to do some of his own: a sculpted column for each side of the saloon and some colored panels in the deck head, which are subtle but distinctive. The whole area is light and bright, with limed oak joinery, cream carpets and headliners, and white leather upholstery; a pale scheme offset by great views out the windows and a set of colorful dining chairs.

Head down past the port side of the helm, and a surprise awaits. Instead of a companionway leading down to the lower accommodations, you find a wholly unexpected extra space on its own deck level, which Pershing designers call the mezzanine. On our boat the area was fitted out with leather chairs and sofas, a sound system, and a huge plasma TV, making it a perfect entertainment suite. It could be equally well fitted out as an office, den, crew mess—you name it, Pershing's design team will be happy to comply.

The owner's suite is directly below, a huge and well-organized space stretched across the full beam of the hull, between those tall topside windows. It is ingeniously designed around diagonals, with two MSD and shower compartments, a big chest of drawers and sideboard, a walk-in wardrobe, and a sofa, arranged unobtrusively around the edges while your eye is still marveling at the distance between the door and the farthest corner. It's like a magic trick. There is no sense at all that any of these has been allowed to rob an inch from the main cabin.

The principal guest suite in the bow also manages to seem huge. While the decorative themes in these areas echo those of the saloon, the two twin guest cabins between them were apparently styled by the owner's daughters, who decreed red leather upholstery and lots of it. It is certainly bold.

Nowhere in the accommodation areas aboard the 115 do you feel that space or volume has been compromised. The cabins are all generously sized and lack neither headroom nor stowage space. The four-berth, three-cabin crew quarters and galley area between the owner's suite and engine room are also well-proportioned, and the engine room itself is vast: deep and wide. Nothing short of a cathedral is ever going to make two MTU Series 4000 V-16s look small, but the engines have plenty of space around them, even with the gensets stacked in the middle. And in the aft section there is that domed pod, ready for the installation of a TF50 gas turbine and third jetdrive should a future owner ever feel the need.

Before I closed my notebook, I made one final discovery, up a hidden spiral companionway from the starboard side of the 115's cockpit: a secluded, recessed, rooftop sunpad that provides you with a completely private lounging area—unless, of course, you're moored next to something bigger. If you were to ask for an upper helm station to be installed, I'm sure Antonelli would be more than happy to listen.

After our test routines and handling trials in a calm Gulf of La Spezia and a 45-mile, high-speed cruise through steepening, quartering seas, we headed northwest by west to Genoa. It was while I was up here enjoying the view that I succumbed to the nagging temptation that had surfaced as we set out. As the 115's debonair French skipper eased his precious charge stern-first into the boat show's most awkward berth, I got out my calculator and started punching keys.

Just how many feet had we been traveling, at something like 40 knots, on each gallon? I checked and rechecked the sum, and then suppressed a guilty shudder: The answer was 692.